Water scarcity has an adverse impact on the global production of food, timber and fuel wood, as well as on the terrestrial sequestration of carbon in plants. The situation is expected to worsen as global climate change disturbs the precipitation pattern around the world. Water shortage and scarcity are therefore a priority issue of major global concern. However, water vapor present in air may represent an abundant source of clean drinking water if it could be harvested efficiently and inexpensively.
Fog is common in many coastal, high-altitude and forested regions of the world and therefore forms an important source of freshwater. For example, water is harvested from fog along the Pacific coast of South America, the Atlantic coast of North Africa and the coast of Cape of Good Hope in South Africa. Simple mesh nets are used to collect condensed water droplets from fog, and 5 to 20 L/m2/day of water collected has been reported depending on the net material and design, location and prevailing environmental conditions. Such a technology is inexpensive as it requires little or no maintenance.
Moisture collection and droplet flow may also be performed by means of bio-materials inspired from nature, using mechanisms that mimic the ones used by plants, animals and insects that live in arid and semi-arid regions and therefore, are adapted for water scarcity. For example, a spider web is unique in its ability to collect moisture and accumulate water into large droplets, thus preventing losses from wind and heat. It has been speculated that this is due to the presence of hydrophilic and hydrophobic domains along the silk strand.
In addition to the water harvesting from fog or air, another source of water is the water present in the air as humidity which can be removed by using air conditioning or air dehumidifier systems.
Humidity plays a key role in our perception of thermal comfort. Controlling the humidity level is also important in suppressing microbial growth indoor such as mildews, molds and rots that may damage building structures and generate unwanted bioaerosols (i.e., spores) and toxic microbial emissions that can trigger asthma and allergy, causing respiratory distress and even toxic shock and death in severe cases. More than a third of the electricity used in space conditioning is wasted in cooling water moisture that has large latent and sensible heats compared to air. Indeed, the energy required to lower the humidity level of 1 kg of air from 75 percent relative humidity (RH) to the comfort level of 60 percent RH at a fixed temperature of 25° C. is 9 times more than the energy needed to cool 1 kg of dry air by 1° C. Therefore, a significant energy saving with the concomitant reduction in greenhouse gas emission can be achieved if moisture could be removed from air prior to cooling. However, current refrigeration-cycle and adsorbent-based dehumidification technologies are energy intensive. Therefore, there exists a continuing need for designing materials, methods and systems for moisture removal and/or water harvesting from air.